1. Field of the Invention
The present invention relates to a local oscillator system for radio-frequency (RF) communication systems and a frequency switching method of the oscillator system, and more particularly, to a local oscillator system for RF communication systems with frequency generators each of which generates a plurality of frequencies different from each other, and a frequency switching method of the local oscillator system.
2. Description of the Prior Art
FIG. 1 is a functional block diagram for showing a frequency switching method of a conventional local oscillator system.
In FIG. 1, there are provided a plurality of phase-locked loop (PLL) frequency synthesizers 1, each of which generates a plurality of frequencies from a single reference frequency, and a switch circuit 3 which selects one of a plurality of local signals with a respective frequency to send it to an output terminal 5. Each of the synthesizers 1 sends one of the frequencies generated to the switch circuit 3, and the frequencies sent from all the PLL synthesizers 1 are different from each other and belong to the same band of frequencies. The switch circuit 3 is generally composed of pin diodes for high-speed switching.
With the conventional frequency switching method, the circuit configuration of the local oscillator system is simple, however, there are several problems. A first problem is that all the synthesizers 1 operate simultaneously and isolation between input and output ends of the switch circuit 3 is not sufficient for such frequencies, so that the frequencies not selected by the switch circuit 3 are sent to the output terminal 5 as spurious components.
The reason is that the unwanted or spurious frequencies are easily sent to the output terminal 5 at such high frequencies because the pin diode used in the switch circuit 3 performs its switching operation by changing the electric resistance between its input and output ends and the isolation characteristic between these ends becomes degraded as the input frequency becomes higher.
Since all the frequencies generated by the respective synthesizers 1 belong to the same band of frequencies, it is not possible to remove the spurious components by using ordinary elements such as bandpass filters. As a result, when the tolerance for the spurious components is strict, the conventional frequency switching method shown in FIG. 1 is difficult to be applied to such a conventional local oscillator system.
To solve the problem of spurious components, a pin diode adapted to provide sufficient isolation between the input and output ends at high frequencies may be employed in the switch circuit 3. However, such a pin diode is very expensive, resulting in high fabrication costs.
The second problem with the above-mentioned conventional method is that whenever the local frequencies to be outputted are changed by the switch circuit 3, the loads of the respective synthesizers 1 vary momentarily to cause fluctuation in the frequencies out from the synthesizers 1. As a result, the frequency switching operation is not completed until the fluctuation of the frequencies has stopped. This means that the switching time is made long and a quick switching operation is impossible.
FIG. 2 is a block diagram for showing another conventional frequency switching method of a local oscillator system, in which PLL frequency synthesizers 11, a fixed-frequency oscillator circuit 12, frequency mixer circuits 13, bandpass filters 14 and a switch circuit 23 are provided.
Each of the PLL frequency synthesizers 11 generates first signals with different frequencies belonging to the same band of frequencies and outputs one of them to the corresponding mixer circuit 13. The oscillator circuit 12 generates a second signal with a single, fixed frequency and outputs it to all the mixer circuits 13.
The frequencies from the synthesizers 11 and the fixed frequency from the oscillator 12 have the following relationship. The sums or differences between each of the frequencies from the synthesizers 11 and the fixed frequency from the oscillator 12 are equal to desired local frequencies.
Each of the mixer circuits 13 mixes in frequency the first signal output from the corresponding synthesizer 11 and the second signal output from the oscillator circuit 12 to send it to the corresponding bandpass filter 14, resulting in a signal having the sum and difference of the frequencies output from the corresponding synthesizer 11 and the oscillator circuit 12.
Each of the filters 14 removes a component with an undesired frequency, for example, the sum frequency, from the output signal of the corresponding mixer circuit 13 and transmits a component with a desired frequency, for example, the difference frequency, to the switch circuit 23. The spurious components from the mixer circuits 13 are removed by the corresponding filters 14, respectively.
The switch circuit 23 selects one of the output signals with the different frequencies from the bandpass filters 14 and sends it to the output terminal 15 as a desired local signal, similar to the case as shown in FIG. 1.
Thus, with the conventional method shown in FIG. 2, the signals with the desired frequencies are sent to the switch circuit 23 after their spurious components have been removed, so that the first problem can be solved. In addition, since the switch circuit 23 is electrically connected to the frequency synthesizers 11 through the corresponding frequency mixer circuits 13 and bandpass filters 14, respectively, the loads of synthesizers 11 do not vary momentarily whenever the local frequency is changed and as a result, the fluctuation of the local frequencies taken out from the output end 15 can be cancelled. This means that the above-mentioned second problem can be also solved.
Yet, with the method shown in FIG. 2, when the input frequencies are higher and the tolerance for the spurious components is strict, the above-mentioned first problem related to the spurious components remains unsolved due to insufficient isolation between the input and output ends of the switch circuit 23.